Diesel engines and, to a lesser extent, other internal combustion engines generate nitrogen oxide (NOx) gasses as byproducts of the fuel combustion process. NOx gasses may be present in an exhaust stream in various forms, including as nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). A selective catalytic reduction (SCR) device is typically used as part of a vehicle exhaust system to reduce NOx gasses before the exhaust is discharged into the atmosphere. The catalytic action of the SCR device and an associated reductant such as ammonia or urea ultimately converts NOx gasses into inert byproducts, i.e., nitrogen and water.
In vehicles having a diesel engine, NOx sensors are typically positioned upstream and downstream of the SCR device. The upstream or front NOx sensor measures NOx levels emitted by the engine, while the downstream/rear NOx sensor measures NOx levels remaining in the exhaust stream after treatment by the SCR device. Together, the front and rear NOx sensor measurements are used to calculate the overall NOx removal efficiency. Because a high degree of confidence is required in the levels of NOx gasses discharged by vehicles, certain government agencies require the calculation and recording of NOx removal efficiency. Additionally, periodic monitoring of the plausibility of any readings from the rear NOx sensor is required to ensure the overall operational integrity of a diesel exhaust system.